Fix seth/rebar_lock_deps_plugin/issues/16.

[efine]

Consider a directory layout like this:

apps/lexer/rebar.config
     parser/rebar.config
     codegen/rebar.config
deps/*/rebar.config
rel/
rebar.config

The top-level rebar.config specifies {sub_dirs, ["rel", "apps/*"]},
and each rebar.config has its own set of deps.

rebar_lock_deps_plugin does not lock the dependencies specified in the
apps/{lexer,parser,codegen}/rebar.config files. This is a major issue
for certain projects of mine.

This difficulty arises because rebar_lock_deps_plugin apparently does
not scan sub_dir directories for dependencies. Since rebar itself
does scan sub_dir directories for dependencies, it seems reasonable
to expect that rebar_lock_deps_plugin would do it, too.

[seth]

indent looks off here? maybe a hard tab?

[seth]

Hiya. Thanks for this PR.

Would you be willing to add a small test case that could be used to verify the behavior? I know the current test coverage isn't great, but no time like the present to improve that :)

A few minor comments and since I merged your other fix this now needs a rebase. But I like the idea of teaching the plugin how to handle nested projects as you have.